This invention relates to spectrometers, in particular to spectrometers for detecting ions based on their mobility.
An ion mobility spectrometer (IMS, also referred to as an ion mobility detector) is a chemical detector whose operation is based on the fact that different ions have different electric charges and masses. As a result, when moving in a reference gas in an electric field, the different ions have different mobility, or velocity, as a result of the electric field. An IMS typically includes an ion source for generating ions, an ionization chamber where ion-molecule reactions occur as a result of samples being bombarded with the generated ions, an ion gate region for directing the ionized samples, also referred to as secondary ions, to be analyzed, an ion drift region to allow the ionized samples to separate so that they may be detected, and an ion collection region for detection and identification of the sample ions. In the ionizer, radioactive materials such as, for example, tritium, Ni, Am, etc. may be used to ionize the samples. An electric field is typically used in the ion gate region to direct the ions into the ion drift region. In the ion drift region, the sample ions are again subjected to an electric field where they separate according to their mobility, as mentioned above.
Unlike a mass spectrometer, which requires a high vacuum, the IMS has the advantage of operating under atmospheric pressure. It can be used as a stand-alone detector using its own analyte separating ability. It also can be used in combination with other analytical techniques. For example, an IMS can be used as a chromatographic detector where analyte separation first takes place in a column mounted upstream of the IMS. The resulting separated output is then directed into an IMS for further analysis. This multiple analysis technique has been used successfully in Gas Chromatography, Liquid Chromatography and Super-Critical Chromatography.
The IMS is a highly sensitive detector with detection limits observed well below ranges in nanograms. However the IMS suffers from a limited dynamic range due to a response decay at higher analyte concentrations. Like other concentration-dependent ionization detectors, the IMS is linear only up to a limit determined primarily by the strength of the ionization source. It is estimated that this limit may be reached when half the ionizing particles are consumed in ionizing analyte molecules. After the limit of the linear response of the IMS, there is a transitional range where the response becomes non-linear. Eventually, at high sample concentrations, the IMS response becomes logarithmic in nature. It is known to use a logarithmic calibration curve to compensate for this logarithmic response.
Reference in this regard may be had to R. H. Hill and D. G. McMinn, xe2x80x9cDetectors for Capillary Chromatography,xe2x80x9d pp. 311-313 (John Wiley and Sons, Inc., 1992)
A disadvantage of the IMS is that the user must often dilute samples in order to work within the linear range.
Another disadvantage is that in the event that the user wishes to use high sample concentrations without dilution, the user must determine the beginning of the logarithmic range independently, as there are no currently available guidelines, and then establish a logarithmic calibration curve specifically for the particular analytes of interest.
An additional disadvantage is that the above mentioned transitional range between the linear range and the logarithmic range is typically not calibrated. Therefore, determining concentrations of analytes falling in this region requires a complex calibration curve.
It is an object and advantage of the invention to provide an ion mobility spectrometer which has a linear response over substantially all of its response curve. It is a further object and advantage of the invention to linearize the output of the IMS in a manner that preserves the linear response range, while linearizing the transitional response range and the logarithmic response range of the IMS response curve, thus relieving the user of complex and tedious calibration procedures.
An ion mobility spectrometer is disclosed which includes a sample input port, an ion generator, an ionization chamber receiving and ionizing samples, an ion gate for causing the ionized samples to travel in a direction, and a drift region for receiving the directed ionized samples and for subjecting the ionized samples to an electric potential. The ionized samples then separate according to their electric charge and mass and are detected by a sensor having an output with linear, non-linear, and logarithmic characteristics. The ion mobility spectrometer further includes circuitry coupled to the sensor for linearizing the output such that the non-linear and logarithmic characteristics are linearized while preserving the linear characteristics. The circuitry operates to linearize the output by multiplying the output by a first function determined from a second function by extrapolating linear and logarithmic characteristics based on a logarithmic plot, dividing the output current by the extrapolation, adding 1, and raising the result to a power determined from a slope of the logarithmic characteristics to obtain a linearizing function. The result is one continuous linear plot in conformance with the fundamental gain of the IMS detector.